Magnetic nanoparticles typically comprise nanocrystals made of oxides (or to a lesser extent of sulfides) of the elements in the forth row of the periodic table (i.e. Cr, Mn, Fe, Co, and Ni). The ability to produce such magnetic nanoparticles is inevitable not only for the general understanding of magnetic properties in a nanometer regime but also for manifold technical applications ranging from magnetic resonance imaging, drug delivery, catalysts, and biosensing to nanoelectronics, semiconductor materials, and magnetic storage media (reviewed in Lu, A. et al. (2007) Angew. Chem. Int. Ed. 46, 1222-1224). Importantly, the magnetic properties of these nanocrystals strongly depend on their dimensions, that is, on their size and shape. For example, larger nanocrystals having a size of >35 nm in diameter have a permanent single magnetic domain (i.e. they are ferromagnetic), whereas smaller particles of <25 nm in diameter exhibit superparamagnetic properties (i.e. they are not permanently magnetic at ambient temperature). In previous years, research has thus mainly focused on the production of “size-adjusted” magnetic nanoparticles having “tailored” magnetic and physicochemical properties.
Traditionally, magnetic nanoparticles are synthesized chemically through precipitation of the crystals from basic aqueous solutions. However, the production of particularly dimensioned nanocrystals via these synthesis routes is significantly hampered by the broad size distribution of the crystal populations obtained. More recently, the synthesis of nanocrystals has been directed to non-aqueous approaches generally resulting in the formation of crystals having not only an improved overall quality but also a narrower size distribution. Nevertheless, in most chemical syntheses reported so far only sub-gram to low gram quantities of monodisperse nanocrystals were obtained, not sufficient for many applications. Furthermore, only a fraction of such synthetic particles constitutes monocrystalline particles having defined magnetic properties. By varying the experimental conditions employed the size of the particles could be controlled to some extent. However, the typical maximal diameters of the resulting particles were only in the range of about 25 nm, which is too small for clinically relevant applications such as magneto-hyperthermic treatment of tumors (Jana, N. R. et al. (2004) Chem. Mater. 16, 3931-3935; Park, J. et al. (2004) Nature Materials 3, 891-895; Hergt, R. et al. (2005) J. Magnetism Magnetic Materials 293, 80-86).
Alternatively, biogenic magnetic nanoparticles can be employed that are produced by magnetotactic organisms, predominantly magnetotactic bacteria. The ability of magnetotactic bacteria to orient in the Earth's magnetic field is based on the presence of specific organelles, the magnetosomes, which are membrane-enveloped monocrystalline crystals (i.e. crystals having a single magnetic domain) of a magnetic mineral that are arranged in chain-like structures within the cell. Magnetosomes display a variety of species-specific shapes within the single magnetic domain size range (reviewed in Bazylinski, D. A. and Frankel, R. B. (2004) Nature Rev. Microbiol. 2, 217-230). In the prototypical Magnetospirillum, cubo-octahedral nanocrystals of the mineral magnetite (Fe3O4) having a maximal diameter of 50 nm are synthesized within magnetosome membrane (MM) vesicles. The MM is a phospholipid bilayer of a distinctive biochemical composition. Different methods for both the cultivation of magnetotactic bacteria and the isolation of magnetosomes thereof are well established in the art (U.S. Pat. Nos. 4,385,119 and 6,251,365; Heyen, U. and Schüler, D. (2003) Appl. Microbial. Biotechnol. 61, 536-544).
In Magnetospirillum (M.) gryphiswaldense approximately 20 magneto some membrane proteins (MMPs) were identified so far, which are supposed to be involved in magnetosome biomineralization. However, the individual functions of MMPs have remained largely unknown, and only few magnetosome proteins are currently characterized in greater detail. Based on the data available magnetosome formation appears a highly complex process with strict control over MM-vesicle differentiation and formation, iron transport as well as nucleation, growth, and assembly of the magnetite crystals into chain-like structures. In order to function effectively in magnetic orientation, crystal sizes have to be controlled precisely within the single magnetic domain range, as the magnetic properties of magnetic nanocrystals change drastically with the particle's dimensions (see above). In previous studies, it was shown that decreasing the iron concentration in the growth medium or an increase in oxygen pressure resulted in the formation of smaller magnetite nanocrystals than under normal growth conditions. However, changing such environmental parameters does not allow for a reliable and accurate control of crystal size and shape. Rather recently, the isolation of spontaneous M. gryphiswaldense mutants producing smaller and aberrantly-shaped particles than wild-type cells (Hoell, A. et al. (2004) Phys. B. 350, e309-e313; Ullrich, S. et al. (2005) J. Bacterial. 187, 7176-7184) indicated that crystal dimensions are under genetic control as well. However, it is currently unknown how this regulation is achieved at the molecular level, and least of all what are the genetic factors the control of whose expression would enable the synthesis of “size-adjusted” magnetosomes.
Thus, there still remains a need for methods for producing magnetic nanoparticles that overcome the above-mentioned limitations. In particular, there is a need for methods enabling the reliable controlled production of monocrystalline particles having a (pre-determined) defined size suitable for a given application not only in high quantities but also in an easy-to-do and cost-efficient manner not requiring any sophisticated instrumentation and/or specific reactants.
Furthermore, there is also a need for corresponding monocrystalline magnetic nanoparticles having a defined size and thus displaying specific magnetic and/or physicochemical properties. In particular, there is a need for “customized” magnetic nanoparticles whose size and shape are specifically adapted for a particular use.
Accordingly, it is an object of the present invention to provide such “size-adjusted” magnetic nanoparticles as well as methods for their production.
These goals are accomplished by the recombinant magnetic nanoparticles and the method for producing the same as defined in the present invention.